This invention relates to an electrochemical cell of the type having a tubular outer electrode surrounding an inner electrode to define therewith an annular passage through which electrolyte solution can be passed.
Electrochemical cells of this type have electrodes which define a passage having a constant cross section over the whole of the length of the passage. The flow of electrolyte through the passage, particularly adjacent to the ends of the electrodes, is turbulent.
Some applications of such cells involve the use of aqueous electrolyte solutions, for example sodium chloride solution for the production of sodium hypochlorite for water purification. In many instances, such solutions contain calcium ions, particularly in areas in which the water is hard, which results in the deposition of calcium salts on the electrodes. The deposition of calcium salts occurs preferentially in the regions in which turbulent flow occurs. The deposition of calcium salts results from an electrochemical process involving the formation of calcium hydroxide at the electrode which forms the cathode of the cell.
Some deposition of insoluble calcium carbonate also occurs thoughout the whole of the cell by the decomposition of soluble calcium bicarbonate and by the combination of calcium hydroxide with atmospheric carbon dioxide.
Such cells can become choked by the deposited salts in a relatively short time, for example within a few weeks or even less in areas in which the water is very hard.
The rate at which the deposition of the calcium salts occurs can be minimized by the use of additives to the solution. However, when the treated water is intended for human consumption or use, the presence of the additives is undesirable and may not be permitted.
Alternatively, the rate of build-up of the deposited salts can be minimized by reversing the electrical current passing through the cells so that gas generated at the anode (previously the cathode) results in the deposited salts being flushed away. However, to be effective such current reversal has to occur say every 10 to 15 minutes. It has been found that current reversal has a detrimental effect on the thin platinum coating of the electrodes and that, consequently, such a high frequency of current reversal severely limits the life of the electrodes, for example, about 6 months average for electrodes having a 7.5-micron-thick platinum coating.